1. Field of the Invention
The present invention relates to an electronic watermark embedment apparatus embedding information in image data in a humanly imperceptible form and generating the image data embedded with a watermark for printing, and to an electronic watermark detection apparatus detecting embedded information on the basis of a scanned image obtained from printed matter containing image data embedded with a watermark.
2. Description of the Related Art
An electronic watermark is a technique for embedding the tracking information, such as copyright information and the person who performed the printing, in electronic data such as image and voice. An electronic watermarking technique is for detecting information embedded as a “watermark” from electronic data, thereby making it possible to confirm an “unauthorized duplication” of a literary work and track leakage of “internal use only” information. Particularly for an electronic medium that allows printing, such as a document image, it is necessary to have the capability of detecting watermark information not only from electronic data but also from a printed paper medium.
Known watermark techniques include a method (e.g., refer to non-patent document 1 listed below) for embedding information by changing the character interval of an English character string, and a method (e.g., refer to non-patent document 2 listed below) for embedding information by enlarging, reducing or rotating a character.
Also, as a method for embedding information in the background of a document, a method is disclosed (e.g., refer to patent documents 1 and 2 listed below) for embedding a dot pattern in the background of a document image. By embedding a dot pattern in the background of a document image, it is possible to detect information stably even if a printed document is somewhat dirty.
Furthermore, as a method for embedding information elsewhere other than the background of a printed document, the disclosed method includes a method (e.g., refer to patent document 3 listed below) for providing a judgment criteria and a priority order for an alteration on the basis of the human sense of sight related to a binary image and applying an alteration starting from a pixel with a high priority and proceeding in order of priority, thereby inserting watermark information; and a technique (e.g., refer to patent document 4 listed below) of extracting a feature point from a character in a document image, thereby embedding an electronic watermark in the feature point.
Patent document 5 listed below relates to a method for embedding watermark information in a black & white binary document image, while patent document 6 listed below relates to a data embedding method for identifying an embedded position by converting a binary image to a multiple value image. Meanwhile, non-patent document 3 shows that the human sense of sight has a tendency to perceive a proximity area, a closed area and a continuous area as a single unified area.    Patent document 1: Registered Japanese patent No. 3628312    Patent document 2: Laid-Open Japanese Patent Application Publication No. 2001-346032    Patent document 3: Laid-Open Japanese Patent Application Publication No. 2004-289783    Patent document 4: Laid-Open Japanese Patent Application Publication No. 2006-074166    Patent document 5: Laid-Open Japanese Patent Application Publication No. 2001-078006    Patent document 6: Laid-Open Japanese Patent Application Publication No. 2006-108994    Non-Patent document 1: J. T. Brassil, S. Low, N. F. Maxemchuk, and L. O'Gorman, “Electronic marking and identification techniques to discourage document copying”, Proc. IEEE INFOCOM '94, vol. 3, pp. 1278-1287, 1994    Non-Patent document 2: Yasuhiro Nakamura and Kineo Matsui (co-authors), “Embedding signature information into Japanese printed documents for copyright protection”, the 50th Annual Conference of the Information Processing Society of Japan; vol. 3, pp. 203 to 204, 1994    Non-Patent document 3: Eiji Yodogawa, Yoichi Higashikura and Kazunari Nakane (co-authors), “Recognition science on the senses of sight and hearing”, pp. 18 through 21; edited by the Institute of Electronics, Information and Communication Engineers (Japan); Corona Publishing Co. Ltd.; 1998
The conventional methods for embedding an electronic watermark noted above are faced with the following problems.
The embedment method as noted in non-patent documents 1 and 2, being capable of embedding only in the characters within a document, are faced with the problem that the amount of information to be embedded largely depends on the number of characters.
The embedment method as noted in patent documents 1 and 2 is faced with the problem that information that has been embedded can easily be recognized because the dot pattern embedded in the background is rather conspicuous.
The embedment methods as noted in patent documents 3 and 4 embed information especially in the contours and gaps of figures and characters, and therefore are faced with the problem that there is a risk of losing the embedded information when the material is printed or copied a few times and the image becomes blurry in, for example, the contours.